Cell identifier conflict avoidance

ABSTRACT

The technology described automatically resolves cell identity collisions/conflicts in a cellular radio communications network. A detecting node determines that a first cell identifier associated with a first conflicting cell is the same as a second cell identifier associated with a second conflicting cell. One of the first and second conflicting cells is selected to change its cell identifier. A different cell identifier is determined for the selected cell. The different cell identifier is then provided to other cells and preferably to user equipment (UE) terminals without disrupting ongoing UE communications.

RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 11/838,770, filed on Aug. 14, 2007, which is related tocommonly-assigned U.S. patent application Ser. No. 11/838,753, filed onAug. 14, 2007 (now U.S. Pat. No. 8,559,952). The above identifiedapplications and patent are incorporated by reference.

TECHNICAL FIELD

The technical field relates to mobile radio communications involvingmobile radio terminals and radio base stations in a mobile radiocommunications system.

BACKGROUND

Universal Mobile Telecommunications System (UMTS) is a 3rd Generation(3G) mobile communication system employing Wideband Code DivisionMultiple Access (WCDMA) technology standardized within the 3^(rd)Generation Partnership Project (3GPP). In the 3GPP release 99, the radionetwork controller (RNC) controls resources and user mobility. Resourcecontrol includes admission control, congestion control, and channelswitching which corresponds to changing the data rate of a connection.

The Long Term Evolution (LTE) of UMTS is under development by the 3rdGeneration Partnership Project (3GPP) which standardizes UMTS. There aremany technical specifications hosted at the 3GPP website relating toEvolved Universal Terrestrial Radio Access (E-UTRA) and EvolvedUniversal Terrestrial Radio Access Network (E-UTRAN), e.g., 3GPP TS36.300. The objective of the LTE standardization work is to develop aframework for the evolution of the 3GPP radio-access technology towardsa high-data-rate, low-latency and packet-optimized radio-accesstechnology. In particular, LTE aims to support services provided fromthe packet switched (PS)-domain. A key goal of the 3GPP LTE technologyis to enable high-speed packet communications at or above about 100Mbps.

FIG. 1 illustrates an example of an LTE type mobile communicationssystem 10. An E-UTRAN 12 includes E-UTRAN NodeBs (eNBs) 18 that provideE-UTRA user plane and control plane protocol terminations towards theuser equipment (UE) terminals 20 over a radio interface. An eNB issometimes more generally referred to as a base station, and a UE issometimes referred to as a mobile radio terminal or a mobile station.

Each base station transmits a signature sequence over an entire cellarea for the UE terminals to detect and measure. Measurements performedby the UE terminals on the received signal strength of different basestation signature sequences are used in most radio communication systems(e.g. GSM, WCDMA, LTE, WCDMA-2000 etc.) to perform, e.g., initial cellselection and handover decisions. A signature sequence in WCDMA includesa particular scrambling code that is applied to the common pilot channeltransmitted from each NodeB. The WCDMA standard specifies 512 uniquescrambling codes with 512 corresponding MCIs. In LTE, a signaturesequence is two-dimensional and is generated as a symbol-by-symbolproduct of a two-dimensional orthogonal sequence and a two-dimensionalpseudo-random sequence. In total, the LTE standard defines 510 suchunique signature sequences with 510 corresponding MCIs. In LTE, UEsmeasure the signature sequence for neighboring cells to determine areference symbol received power (RSRP), and these RSRP measurements areused when performing initial cell selection for UEs to “camp” on as wellas when performing handovers of UE connections.

Ideally, the signature sequences that a single UE can detect areuniquely mapped to a particular base station. But in most radiocommunication systems, the number of unique signature sequences that aparticular standard specifies is less than the number of base stationsin the system. The number of signature sequences is limited becausetransmission of a signature sequence is associated with a radio resourcecost, i.e., power, bandwidth, code space, frequency space, or time, andthat cost increases with the number of unique signature sequences forwhich the system is designed. Another reason why the number of signaturesequences is limited relates to the UE mobile stations frequentlyreporting measurements related to the different signature sequences backto the radio network, e.g., to the serving base station. A UE may reportseveral such measurements several times per second, and therefore, it isdesirable that such measurement reports can be encoded with fewer bitsto reduce the impact of those reports on the limited radio bandwidth.

In light of these considerations, a one-to-one mapping may beestablished between a signature sequence transmitted by the base stationand a measurement cell identity (MCI) used by the UEs in the encodedmeasurement reports. The term MCI is used here as a convenient way ofspecifying which particular signature sequence a given base station istransmitting. An MCI may be viewed as an index that permits the UE todetermine the corresponding signature sequence.

UEs continuously monitor system information as well as the signaturesequences broadcasted by base stations within range to inform themselvesabout “candidate” base stations in the service area. When a UE needsaccess to services from a radio access network, it sends a request overa random access channel (RACH) to a suitable base station, typically abase station with the most favorable radio conditions. As shown in FIG.1, the base stations are interconnected with each other by means of anX2 interface. The base stations are also connected by an S1 interface toan Evolved Packet Core (EPC) 14 which includes a Mobility ManagementEntity (MME) by an S1-MME and to a System Architecture Evolution (SAE)Gateway by an S1-U. The MME/SAE Gateway is as a single node 22 in thisexample. The S1 interface supports a many-to-many relation betweenMMEs/SAE Gateways and eNBs. The E-UTRAN 12 and EPC 14 together form aPublic Land Mobile Network (PLMN). The MMEs/SAE Gateways 22 areconnected to directly or indirectly to the Internet 16 and to othernetworks.

One important focus area in LTE/SAE standardization work is to ensurethat the evolved network is simple to deploy and cost efficient tooperate. The vision is that the evolved network will be self-optimizingand self-configuring in as many aspects as possible. Aself-configuration process is one where newly-deployed nodes areconfigured by automatic installation procedures to get the necessarybasic configuration for system operation. A newly-deployed base stationtypically contacts a central server (or several such servers) in thenetwork and obtains configuration parameters needed in order to startoperating. Self-optimization is a process where UE and base stationmeasurements and performance measurements are used to automatically“tune” the network. One example is automating neighbor cell lists, andone non-limiting way of automatically building neighbor cell lists isdescribed in commonly-assigned, U.S. patent application Ser. No.11/538,077, filed on Oct. 3, 2006, and published as US 2007/0097938, thecontents of which are incorporated herein by reference. In GSM andWCDMA, base stations send neighbor cell lists to connected UEs so theyhave a defined set of cell broadcasts to measure (e.g., the signalquality or strength of those broadcasts) to permit determination ofwhich if any neighbor cells is a suitable candidate for handover. In anLTE system, neighbor cell relation (NCR) lists are also used in the eNBsto set up connections over the X2 and/or S1 interfaces.

An area potentially advantageous for self-configuring is automaticassignment of shorter measurement cell identities (MCIs) to basestations. Shorter cell identifiers like an MCI used in the UEmeasurement reports frequently transmitted to the network reduce theamount of radio resources consumed. The shorter cell identifiers aretherefore sometimes referred to here as reporting cell identifiers. Inaddition to the short MCI, each cell is associated with a longer cellidentity that uniquely identifies the cell within the public land mobileradio network (PLMN) to which the cell belongs. A non-limiting exampleof such a longer identifier is a cell identity on the PLMN level (CIPL).

With a limited number of MCIs or other reporting cell identifiers, someMCIs are likely to be reused in larger networks, which means networkplanning is needed. Today, such planning is typically done manually. Forexample, when planning in an LTE RAN, each cell in the network isassigned an MCI, and the planner tries to distribute the MCIs so thatneighboring cells do not have the same MCI. But such attempts may notalways be successful. This is true even if this operation is to beperformed automatically using a suitable allocation algorithmimplemented on a computer. An automatic MCI allocation algorithm shouldpreferably also be capable of assigning MCIs in difficult networksdeployments, e.g., networks with a large number of home base stationsover which the network operator has little control.

In order to perform a handover in LTE from a source cell to a targetcell, the two involved cells must first set up a neighbor cell relation(NCR). The NCR contains at least the MCI (or other short cellidentifier) and the CIPL (or other longer cell identifier). The NCR mayalso include connectivity information such as the IP address of thecorresponding cell, information about the configurations of the X2 andS1 interfaces, and parameters needed for different radio resourcemanagement control algorithms, such as handover thresholds. Informationabout the radio access technology (RAT) (e.g., LTE, WCDMA, or GSM) aswell as other capabilities of the target cell may also be included inthe NCR.

Building the NCR list in each base station can be done automatically.Each time a new base station is deployed, it contacts a central serverin the network and that server assigns the base station with a CIPL andan IP address. The base station may begin operation with an empty NCRlist, and each time it receives a measurement report from a served UEthat contains a MCI that is not included in the NCR, the base stationasks the UE to obtain the CIPL of that corresponding (non-serving) basestation. In LTE, the CIPL is broadcasted (infrequently) on the broadcastchannel (BCH) which allows the UE to detect the corresponding CIPL ofthe non-serving base station and report it back to the serving basestation. The serving base station can then contact the central server toobtain the remaining NCR information corresponding to that non-servingbase station.

When a base station has two neighbors with different CIPLs but with thesame MCI, there is an MCI “collision” or conflict. As a result, one ormore cells must change its old colliding MCI to a non-colliding MCI. Tomake this change requires closing down the cell, reconfiguring the newMCI value, and then restarting the cell. Alternatively, the cell mayjust change the MCI without closing and restarting, which means that allthe UEs currently “camped” on that cell loose synchronization disturbingall active UE communications in that cell. Those disturbed UEs mustperform new cell searches likely resulting in at least most of themselecting that same cell and performing a random access attempt. Such amass random access is problematic because the typical random accesschannel is not designed to handle a large number of simultaneous accessattempts. Alternatively, those UEs could select another, lesssatisfactory cell.

Another problem with such MCI collisions is that all neighboring cellsto the cell with the new MCI no longer have correct and currentinformation in their respective neighbor cell relation (NCR) lists.Consequently, when those neighboring cells receive measurement reportsfrom the UEs using the new MCI, the neighboring cells must thenre-establish their relationship to the cell with the new MCI. Untilthen, the neighboring cells can not order any UEs to perform a handoverto that cell, which could result in dropping those calls that needhandover being dropped.

MCI collisions will cause significant problems when new home or otherbase stations are set up without any coordination in a densely populatedarea (e.g., Manhattan). Each time a consumer sets up a home base stationor moves the location of that home base station, there is a highlikelihood of many MCI collisions because the network operator is not incontrol of that base station set up or movement and therefore can not dothe cell planning/coordination needed to avoid MCI collisions. Duringthe “roll-out” phase of a new network, new cells will be added, and MCIcollisions are also likely to occur as a result. “Relay” base stationsmay also be installed in moving vehicles like cars, buses, and trains.Because these base stations move around, frequent MCI collisions may beexperienced. Also, other autonomous changes in a self-organizingnetwork, like adjustments in power levels or in antenna tilt, may causeMCI collisions to occur.

Most of the MCI collisions manifest themselves as ambiguities in NCRlists, i.e., two cells with different CIPLs but with the same MCI arelisted in the NCR list. However, it is also possible that a UEexperiences acceptable radio conditions to the serving cell and thecandidate cell and that both use the same MCI. In that case, the UE maynot be able to report the weaker cell as a candidate cell to the servingcell because the weaker cell is directly interfered by the serving cellon the same signature sequence. To intuitively explain why this is thecase, consider two different scenarios.

In the first scenario, only the serving cell is transmitting a signaturesequence. The UE searches for this signature sequence bycross-correlating a received signal with the same signature sequence. Ifthe received signal contains the corresponding signature sequence, thenthe cross correlation output contains a distinct peak. The location ofthe peak provides the UE with time synchronization, and the amplitude ofthe peak is proportional to the signal strength of the correspondingcell. But due to multi-path propagation in the radio channel, the UEreceives multiple copies of the transmitted signature sequence that havedifferent delays, amplitudes, and phase shifts. There may for example bea direct signal path from the transmitter in the cell and the UE and anindirect reflected signal path that arrives at the UE sometime later. Inthis case, the cross correlation operation in the UE produces twodistinct peaks for the same signature sequence, one per radio path. TheUE uses this correlation peak information to perform channel estimationand equalization.

Now consider a second scenario without multi-path, but with twodifferent cells transmitting the same signature sequence. Thetransmitted signals from the two cells will arrive at the UE withslightly different delays, amplitudes, and phase shifts. The UE performsa cross correlation with the received signal and the output contains twodistinct peaks for the same signature sequence, but in this case, onepeak per cell. What these two scenarios demonstrate is that the UE cannot differentiate between normal multi-path in the radio channel and thesame signature sequence being transmitted from two different cells.

Thus, it is desirable to provide an automated approach to resolve cellidentifier collisions in an efficient fashion as well as to be able todisseminate changed or new cell identifiers in a seamless, automated,and coordinated fashion.

SUMMARY

The technology described automatically resolves cell identitycollisions/conflicts in a cellular radio communications network. Adetecting node determines that a first cell identifier associated with afirst conflicting cell is the same as a second cell identifierassociated with a second conflicting cell. One of the first and secondconflicting cells is selected to change its cell identifier. A differentcell identifier is determined for the selected cell. The different cellidentifier is then provided to other cells and preferably to userequipment (UE) terminals without disrupting ongoing UE communications.For example, a cell identifier change message may be sent to the one ormore other cells. That message can include a time parameter from areceiving cell to indicate when to change the cell identifier for theselected cell to the different cell identifier.

The technology may be implemented in one or more radio network nodes. Ina preferred example embodiment, the functions are distributed amongmultiple radio network nodes, where a cell is associated with a basestation. The term “cell” is used to refer both to the cell coverage areaand to the base station that controls operations in that coverage area.For example, a detecting cell may perform the detecting and selectingtasks, and the selected cell may perform the determining and providingtasks. Moreover, each cell preferably maintains a neighbor cellrelations list that includes each cell's identifier along with otherinformation.

The cell identifier collision detecting operation may be facilitated byreceiving information from one or more user equipment (UE) terminalsregarding transmissions received from the first and second cellsincluding the same cell identifier. Alternatively, the cell identifiercollision detecting operation may include receiving informationregarding cell identifiers for other cells from other network nodes,e.g., other base stations. The network node may also determine aneighbor cell relations (NCR) information from one or more cellsneighboring the selected cell. Based on the NCR information, the networknode can determine a different cell identifier that is different fromcell identifiers indicated in the NCR information.

A preferred (but not essential) feature relates to locking thenon-selected one of the first and second cells to prevent cellidentifier information from being changed at the non-selected cell.After the selected cell changes its cell identifier to the differentcell identifier, the non-selected cell is unlocked. Additional lockingfeatures include detecting that the non-selected cell is already in alocked state, subsequently detecting that the non-selected cell is in anunlocked state, sending a cell identifier locking message to thenon-selected one of the first and second cells to prevent cellidentifier information from being changed at the non-selected cell, andsending a cell identifier change message to the selected cell.Preferably, a determination is made whether the detected cell identifierconflict was resolved before sending the cell identifier locking messageto the non-selected cell.

Another locking feature includes detecting that the selected cell is ina locked state preventing change of cell identifier information for theselected cell, subsequently detecting that the selected cell is in anunlocked state, sending a cell identifier locking message to thenon-selected one of the first and second cells to prevent cellidentifier information from being changed at the non-selected cell, andsending a cell identifier change message to the selected cell. Again, adetermination is preferably made whether the detected cell identifierconflict was resolved before sending the cell identifier locking messageto the non-selected cell.

The first and second cell identifiers can be reporting cell identifiersused by the UE terminals to identify cells associated with a reportingparameter provided by the UE terminals in measurement reports sent bythe UE terminals to the cellular radio communications network. Forexample, if the cellular radio communications network is a long termevolution (LTE) network, the first and second cell identifiers can bemeasuring cell identifiers (MCIs).

If the first and second conflicting cells use the same MCI, then the UEterminals are unable to detect, measure, and report the secondconflicting cell in the handover measurements to the first conflictingcell. The reason is that the UEs are unable to differentiate betweennormal multi-path of the radio channel and “artificial” multi-pathcaused by several cells transmitting the same MCI/signature sequencesignal as explained in the background. To allow the UE to resolve thisambiguity, the first cell broadcasts a transmission gap message at apredefined time in the future. At that time, the first cell does nottransmit its MCI/signature sequence, i.e., during the transmission gap,the UE searches for that MCI/signature sequence associated with thefirst cell. If one of the UEs detects the first cell's MCI/signaturesequence during the transmission gap, then the UE knows that the samesignature sequence is being transmitted by another cell in the system inaddition to the first cell.

In an example transmission gap procedure, the first cell informs one ormore served UE terminals about an upcoming MCI/signature sequence itstransmission gap and requests that the one or more UE terminal(s) try todetect whether other cells are using the first cell's MCI during thattransmission gap. In case the one or more UE terminal(s) detects adifferent cell using the first cell's MCI during the transmission gap,then they report this back to the first cell. The first cell will theninitiate an MCI changing procedure to resolve the MCI conflict.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a block diagram of an example LTE mobile radio communicationssystem;

FIG. 2 a block diagram of an example more general RAN mobile radiocommunications system;

FIG. 3 is a diagram illustrating an example base station servingmultiple cells and broadcasting a corresponding cell signature sequencein each cell area;

FIGS. 4A-4C illustrate example cell identifier collision situations;

FIG. 5 is a flowchart diagram illustrating non-limiting, exampleprocedures for automatically resolving a cell identifier collision;

FIG. 6 illustrates conceptually a transmission gap; and

FIG. 7 is a signaling diagram illustrating non-limiting, examplesignaling messages for issuing transmission gaps;

FIG. 8 is a signaling diagram illustrating non-limiting, examplesignaling messages for automatically resolving a cell identifiercollision and seamlessly changing a cell identifier in accordance with afirst non-limiting, example embodiment;

FIG. 9 is a signaling diagram illustrating non-limiting, examplesignaling messages without locking procedures where the MCI list ofentries in the serving cell NCR list are included in an initial MCIchange message;

FIGS. 10A-10D illustrate an example cell identifier collisionresolution;

FIG. 11 is a signaling diagram illustrating non-limiting, examplesignaling messages for automatically resolving an MCI collision when anon-selected conflicting cell rejects an MCI lock request due to apending MCI lock;

FIG. 12 is a signaling diagram illustrating non-limiting, examplesignaling messages for automatically resolving an MCI collision when aselected conflicting cell rejects an MCI lock request due to an MCIlock;

FIG. 13 is a function block diagram illustrating a non-limiting, examplebase station;

FIG. 14 is a function block diagram illustrating a non-limiting, exampleUE terminal; and

FIG. 15 is a diagram illustrating non-limiting, example signalingroutes.

DETAILED DESCRIPTION

In the following description, for purposes of explanation andnon-limitation, specific details are set forth, such as particularnodes, functional entities, techniques, protocols, standards, etc. inorder to provide an understanding of the described technology. In otherinstances, detailed descriptions of well-known methods, devices,techniques, etc. are omitted so as not to obscure the description withunnecessary detail.

It will be appreciated by those skilled in the art that block diagramsherein can represent conceptual views of illustrative circuitryembodying the principles of the technology. Similarly, it will beappreciated that any flow charts, state transition diagrams, pseudocode,and the like represent various processes which may be embodied incomputer readable medium and so executed by a computer or processor,whether or not such computer or processor is explicitly shown.

The functions of the various elements including functional blockslabeled as “processors” or “controllers” may be provided through the useof dedicated hardware as well as hardware capable of executing softwarein association with appropriate software. When provided by a processor,the functions may be provided by a single dedicated processor, by asingle shared processor, or by a plurality of individual processors,some of which may be shared or distributed. Moreover, explicit use ofthe term “processor” or “controller” should not be construed to referexclusively to a general or special purpose computer capable ofexecuting software code, and may include, without limitation, digitalsignal processor (DSP) hardware, application specific integrated circuithardware (ASIC), read only memory (ROM) for storing software, randomaccess memory (RAM), and non-volatile storage.

It will be apparent to one skilled in the art that other embodiments maybe practiced apart from the specific details disclosed below. Allstatements reciting principles, aspects, and embodiments, as well asspecific examples, are intended to encompass both structural andfunctional equivalents. Such equivalents include both currently knownequivalents as well as equivalents developed in the future, i.e., anyelements developed that perform the same function, regardless ofstructure.

The technology is described in the context of an evolved 3GPP UMTSsystem, such as LTE, in order to provide an example and non-limitingcontext for explanation. But this technology may be used in any moderncellular communications system that supports cell identification. Onenon-limiting example of a general cellular communications system 30 isshown FIG. 2. A radio access network (RAN) 32 is coupled to one or moreother networks 38 such as one or more core network nodes and one or moreexternal networks such as the public switched telephone network (PSTN)and the Internet. The RAN 32 includes base stations 34 that communicatewith each other, e.g., for handover and other coordinated functions. Thebase stations communicate over the radio/air interface with mobile radioterminals also referred to as user equipment terminals (UEs) 36.Although an MCI is used in the LTE context as an example of a cellidentifier, the technology described here may be applied to any cellidentifier.

As described above, each base station broadcasts a predetermined“signature sequence” or other identifier over a known frequency that maybe detected by UEs scanning for such base station broadcasts in a cellarea associated with the broadcast. The term “cell” refers to thegeographical area where an associated base station or eNB provides UEsradio service. But a cell is also sometimes used as a shorthand way ofreferring to the base station or eNB associated with that cell. Eachsignature sequence, which is detectable by UEs, is mapped to arelatively short cell identifier that is used by the UEs when sendingback frequent measurement reports to a serving cell. FIG. 3 shows anexample base station serving three cells 1-3. Each cell transmits itsown signature sequence. Other base stations may only have one cell or adifferent number of cells. Regardless, each cell's own signaturesequence is mapped to a corresponding, relatively short cell identifier.It is typically these shorter, reporting or measurement cell identifiersthat are subject to collisions, although the technology described may beused to resolve any cell identifier collision or conflict. A longer,more unique cell identifier may also be mapped to the short cellidentifier/cell signature sequence.

FIG. 4 illustrates three non-limiting example reporting or measurementcell identifier collision situations. When a collision is detected, thedetecting cell determines which of the conflicting cells will change itsMCI. In FIG. 4A, a nearby cell for the UE shown in the figure has thesame MCI=17 as a serving cell currently serving the UE. The serving cellhas a neighbor cell relation (NCR) that includes cells with MCIs of 5,6, and 15. In this case, the UE terminal may not be able to detect,measure, and report the candidate cell with MCI=17 to the serving cellbecause its MCI is the same as the serving cell. Instead, the nearbycell can be detected if the serving cell effects a transmission gapduring which it does not broadcast its MCI/signature sequence, duringthat gap, the UE terminals in that serving cell can detect if nearbycells are using the same MCI as the serving cell. Upon such detection,the UE9s) inform the serving cell, and at that point, the serving cellis both the detecting cell and one of the conflicting cells. FIG. 4Bshows the candidate cell with the same MCI (MCI=17) as “one” cell in theserving cell neighbor cell relation (NCR) list including MCIs of 5, 6,and 17, but a different CIPL. The detecting cell is the serving cell(MCI=15), and the candidate cell and the one cell in the serving cell'sNCR list including MCIs of 5, 6, and 17 are conflicting cells. FIG. 4Cillustrates the serving cell aiming to add the candidate cell (MCI=5) toits NCR list, but the candidate cell detects an MCI conflict as a resultof the new mutual relation because the serving cell's MCI already existsfor “one” cell in the candidate cell's NCR list including MCI's of 5, 6,and 17. The detecting cell is the candidate cell, and the conflictingcells are the serving cell and the one cell in the candidate NCR list.

FIG. 5 is a flowchart diagram illustrating non-limiting, exampleprocedures for automatically resolving a cell identifier collision. Instep S1, a cell identifier collision is detected between colliding cellsC1 and C2. One of the colliding cells C1 or C2 is selected to change itscell identifier to a different cell identifier (step S2). A different,hopefully non-colliding cell identifier is determined for the selectedcell (step S3). The different cell identifier is provided to one or moreother cells, e.g., cells neighboring or in the vicinity of the selectedcell (step S4). The different cell identifier is also preferablyprovided to UE terminals monitoring the selected cell.

One non-limiting and example way to detect a collision is now described.Each measurement report from a UE contains information about at leastone handover cell candidate including that candidate cell's MCI. If oneor more certain conditions are met, the serving cell may order a UE todetermine and report a CIPL corresponding to a reported MCI.Non-limiting examples of such condition(s) include a handover failureratio is above a threshold for a neighbor with a certain MCI, a periodicMCI check timer has expired, or a central node has ordered CIPLmeasurements for all or selected handovers, etc. Each cell candidate inthe report is processed according to the following. A determination ismade whether the MCI of the candidate cell is a member of the servingcell NCR list. If it is, a determination is made whether the CIPL of thealternative cell and the NCR list entry are the same. If so, a handoverdecision procedure is initiated. Otherwise, the MCI collision must beresolved. If the MCI of the candidate cell is not a member of theserving cell NCR list, then the candidate cell is considered as an NCRlist candidate. A message is then sent over an appropriate interface(e.g., X2 or S1) proposing a mutual neighbor relation between the twocells. If the candidate cell confirms the proposed NCR list update, thenthe candidate cell is added to the NCR list, relevant information aboutthe cell is stored, and a handover decision procedure is initiated. Ifthe candidate cell does not confirm the proposed NCR list update due toan MCI conflict in the candidate cell NCR list addition procedure, theninitiate a candidate cell MCI collision resolution procedure isinitiated.

If a collision is detected, then the detecting cell determines whichcell out of the conflicting cells that will change MCI. For example, thedetecting cell may select the conflicting cell that needs to change MCIusing on one or more of the following guides: the conflicting cell withthe lowest CIPL will change MCI, the conflicting cell with the shortestNCR list will change MCI, the conflicting cell that most recentlychanged MCI will change MCI, the youngest cell will change MCI, the cellwith the lowest cell type will change MCI, and/or the cell using asmallest maximum power will change MCI. One or more other selectingparameter(s) may be used. Indeed, the selection may even be random.

MCI conflicts where the serving cell is one of the conflicting cellscannot be detected just by the serving cell receiving handovermeasurements from served UEs. But such MCI conflicts with the servingcell may be detected by a third cell which detects the MCI conflictbased on handover measurements and NCR list management in the third cellas described above. Alternatively, the serving cell may observe one ormore transmission gaps during which it avoids transmission of itsMCI/signature sequence. An example gap is conceptually illustrated inFIG. 6. The gap may be a configurable time sufficient to allow UEterminals to detect other cells using the serving cell's MCI. Thetransmission gap may be triggered by a node as part of networkmanagement tasks or at random, preferably using a cell specific seed(e.g. based on CIPL), to avoid conflicting cells from performingtransmission gaps at the same time. Alternatively, a gap may betriggered based on observations of unexpected behavior for a particularcell, e.g., dropped calls, poorer radio conditions downlink as comparedto the uplink, etc. Transmission gaps are preferably synchronized withUE MCI measurements.

One example signaling diagram that implements the transmission gap MCImeasurement is shown in FIG. 7. A TRANS_GAP_NOTIFICATION message is sentto served UEs by the serving cell which preferably returnacknowledgement messages. During the transmission gap, the notified UEsmake MCI measurements. Thereafter, the UEs each send anMCI_DETECTION_NOTIFICATION message to the serving cell that containsinformation regarding any MCI conflict detected between the serving cellMCI and an MCI from neighboring cells.

FIG. 8 is a signaling diagram illustrating non-limiting, examplesignaling messages for automatically resolving a cell identifiercollision and seamlessly changing a cell identifier in accordance with afirst non-limiting, example embodiment. In this example, a database typelock is used because multiple base station cell entities can potentiallyconcurrently access each cell's database or table listing MCIinformation. Although MCI changes may performed without such locking,locking prevents MCI data from being corrupted or invalidated whenmultiple cell entities try to write to a cell's database at the sametime. Any single cell entity can only modify MCI information stored inthe database to which it has applied a lock that gives it exclusiveaccess to the stored MCI information until the lock is released. Hence,the first message from the detecting cell entity in Figure is anMCI_LOCK message sent to non-selected conflicting cell 2 that locks cell2 for MCI changes by other cells. Conflicting cell 2 confirms the lockwith an MCI_LOCK_CONFIRM message.

The detecting cell then sends to selected conflicting cell 1 aMCI_CHANGE message requesting that cell 1 change its MCI. The MCI_CHANGEmessage includes the currently used MCI by this conflicting cell 1 sothat cell 1 can detect inconsistencies. Example of such inconsistenciesinclude situations when the MCI change information signaling fails orinformation about previous changes has not yet been communicated to allaffected nodes. If the selected cell 1 is able to change its MCI,initiation of changing that MCI is confirmed by the conflicting cell 1returning a MCI_CHANGE_INITIATED message. The selected conflicting cell1 determines a set of locally-occupied MCI's by requesting the set ofMCI's in the neighbor cell lists of its neighbor cells. The request maybe made by sending a NCR_MCI_REQUEST message to cells in cell l'sneighbor cell list. For this simple example, only neighbor cell relation(NCR) cell 1 is in that neighbor cell list. The NCR cell(s) respond witha list of MCI's corresponding to cells in their neighbor cell list in anNCR_MCI_LIST message. The detecting cell also receives theNCR_MCI_REQUEST message from the conflicting cell 1 and responds withits NCR_MCI_LIST. Alternatively, the detected cell may include its NCRMCI list in the initial MCI_CHANGE message instead. The non-limiting,example signaling diagram in FIG. 9 described below, which is similar tothat in FIG. 8 but without a locking procedure illustrates an initialMCI_CHANGE message that includes the MCI list of entries in the servingcell NCR list.

The NCR_MCI_LIST response message is a list of neighbor cells with oneor more of the following parameters for each neigboor cell: CIPL, MCI,maximum power, cell type, number of neighbor cells, time the cell wastaken into traffic, time the cell was last changed, etc. If one of theconflicting cells has already initiated an MCI changing procedure, thedetecting cell is aware of that because it has received theNCR_MCI_REQUEST message from that initiating cell. In that case, thedetected cell may consider the MCI conflict as being solved or at leastin the process of being solved.

Given the locally conflicting MCI's, the selected conflicting cell 1selects a new MCI among locally vacant MCI's. In order for the selectedcell 1 to determine which MCIs that are locally vacant, it contacts theneighboring cells listed in its NCR list and asks for all MCIs listed intheir corresponding NCR lists. The union of the MCI lists obtained fromneighboring cells defines a set of locally conflicting MCIs for cell 1.Thus, a locally vacant MCIs for cell 1 is any valid MCI that is not inthe list of locally conflicting MCIs for cell 1.

Any locally vacant MCI can be used, and a random MCI selection is shownas a non-limiting example. In case the MCIs are ordered into differentgroups, the selection of a new MCI among the vacant MCIs may berestricted to a certain MCI within the same MCI group. As an example, inLTE the MCIs are organized into 3 groups that correspond to the sametwo-dimensional time-frequency pattern, and 170 identities within eachgroup correspond to different pseudo random sequences. The selection ofthe new MCI may also be restricted to belong to an MCI group other thanthe old conflicting one.

As described in the related application referred to above, the new MCIis broadcast to cells in the neighbor cell list of the selectedconflicting cell 1 using an MCI_CHANGE_NOTIF message and to UE's beingserved by the selected conflicting cell. The message contains (at least)the new MCI and possibly also information about when the new MCI will beput into use. Each of those NCR base stations then preferably respondswith an MCI_CHANGE_NOTIFICATION_CONFIRM message to acknowledge receiptof the MCI_CHANGE_NOTIFICATION message. This acknowledgement message isnot necessary. Such messages may contain an absolute future timereference or a count-down time value so the NCR base stations change atthe same time or approximately at the same time. The MCI changenotification message may be sent over the X2 or the S1 interface to aneighboring base station. When the detecting cell receives theMCI_CHANGE_NOTIF message, it also releases the lock of the non-selectedconflicting cell using an MCI_UNLOCK message. Similar MCI changemessages may be sent to the UEs being served by the selected cell. Inthis way, the MCI change may be accomplished seamlessly, automatically,and in real time with interrupting ongoing UE connections.

FIGS. 10A-10D illustrate an example of a cell identifier collisionresolution. FIG. 10A illustrates an example where the detecting celllocks the candidate cell with an MCI=17 and changes the MCI of theneighboring cell in the detecting cell's NCR list. In FIG. 10B, thechanging cell obtains the MCIs of neighboring cells from its NCR list,which include 5, 6, 29, 39, 56, and 14, and determines that MCI=18 isvacant. Then in FIG. 10C, the changing cell selects vacant MCI=18 andsends an MCI change notification message with its new MCI=18 to thedetecting cell and to its neighbor cells in its NCR list. The detectingcell then sends an unlock message to the non-selected conflicting cellin FIG. 10D.

There is a risk that the initial lock request from the detecting cell tothe non-selected conflicting cell C2 is rejected. One reason is that thecell is already in the process of changing MCI, but the detected cell isunaware (has not yet received the NCR_MCI_REQUEST message). Another isthat the cell is locked for MCI changes. In the first case, the detectedcell considers the conflict to be solved. But in the second case, thedetected cell should wait for the non-selected conflicting cell, whichis locked, to be unlocked.

FIG. 11 is a signaling diagram illustrating non-limiting, examplesignaling messages for automatically resolving an MCI collision when anon-selected conflicting cell rejects an MCI lock request due to apending MCI lock. The detecting cell tries to lock the non-selectedconflicting cell 2 by sending an MCI_LOCK message. But because thenon-selected conflicting cell 2 is already locked, it sends anMCI_LOCK_REJECT message back to the detecting cell. When thenon-selected conflicting cell 2 is released, it sends an MCI_UNLOCK_NOTIF message to let the detecting cell know that it is now unlocked. Thedetecting cell checks to see whether the MCI collision has beenresolved. If not, it sends an MCI_LOCK message to the non-selectedconflicting cell 2 which returns an MCI_LOCK_CONFIRM message. The MCI ofthe selected conflicting cell 1 is then changed as described above, forexample. Alternatively, a cell may obtain and accept several locks fromother cells, and this cell rejects MCI_CHANGE messages until all lockshave been released. Furthermore, each MCI_LOCK message may have anexpiration time to prevent deadlock. Similarly, it is also possible thatthe selected conflicting cell C1 rejects the request to change MCI. Onereason is that the cell is already in the process of changing MCI, butthe detected cell is unaware. Alternatively, the cell is locked for MCIchanges. In the first case, the detected cell may consider the conflictsolved. But in the second case, the detected cell must wait for theselected conflicting cell C2 to be unlocked. FIG. 12 is a signalingdiagram illustrating non-limiting, example signaling messages forautomatically resolving an MCI collision when the selected conflictingcell rejects an MCI lock request due to an MCI lock. The detecting celllocks the non-selected conflicting cell 2 with an MCI_LOCK message andreceives lock confirmation. An MCI_CHANGE message is then sent to theselected conflicting cell C1, but that request is rejected via anMCI_CHANGE_REJECTED message because the selected conflicting cell 1 islocked. Accordingly, the non-selected conflicting cell C2 is unlocked.When the detecting cell later receives an MCI_UNLOCK_NOT IF message fromthe selected conflicting cell C1, it checks to determine whether the MCIconflict was resolved. If not, it sends an MCI_LOCK message to thenon-selected conflicting cell C2 to lock, and after receiving lockconfirmation, the detecting cell sends an MCI_CHANGE message to theselected conflicting cell C1.

FIGS. 13 and 14 are function block diagrams of a non-limiting examplebase station and UE terminal, respectively, that may be used toimplement one or more aspects of this technology. The base station 18,34 in FIG. 13 includes a controller 50, radio equipment 52 coupled totypically multiple antennas, a handover control unit 54, a base stationcommunications interface 56, a core network communications interface 58,a cell list memory 60, and a cell list manager 62. The controller 60supervises the overall operation of the base station, and the otherblocks perform their associated base station functions. The cell listmanager 62 builds and updates a neighboring cell list for this basestation, e.g., an NCR list. The cell list memory 60 provides a mappingbetween a cell's relatively short, reporting cell identifier (RCI),e.g., an MCI, the cell's longer cell identifier, e.g., a CIPL, and otherparameters associated with that neighboring cell. Non-limiting examplesof such parameters include capabilities of the neighboring cell,connectivity information such as IP address and X1/S1 interfaceconfigurations, parameters related to handover (e.g., threshold values,filter parameters, timer values), performance measurement data (e.g.number of successful handovers and total number of handover attempts),or other measurements provided by UEs sending measurement reports to thebase station (e.g. downlink signal strength or quality). The cell listmanager 62 also performs the operations described above for detectingand resolving cell identifier collisions. Regarding collision detection,another option is that the cell list manager 62 may receive informationregarding cell identifier collision from one or more UEs.

The UE terminal 20, 36 in FIG. 14 includes a controller 70, radioequipment 76 coupled to one or more antennas, a signal detector 77, auser interface 72, a measurement controller 78 for measuring signalstrength or signal quality of serving and neighboring base stationcells, a cell list memory 80, and a cell identifier detector 74. Thecontroller 70 supervises the overall operation of the UE terminal, andthe other blocks perform their associated UE terminal functions. Thecell identifier detector 74 determines from transmissions from basestation cells the cell identifier associated with each transmission sothat measurement reports provided to a serving base station include botha cell identifier and one or more measurement parameters. The cellidentifier detector 74 may also determine when the UE receives the samecell identifier from two (or more) cells and provide cell identifiercollision information to the network. Although not necessary, the UE mayalso store and update the UE terminal's neighboring cell list in memorywhich provides a mapping between a cell's relatively short, reportingcell identifier (RCI), e.g., an MCI, the cell's longer cell identifier,e.g., a CIPL, and one or more measured parameters associated with thatneighboring cell detected, measured, or otherwise provided by UEssending measurement reports to the base station, e.g., downlink signalstrength or quality, etc. Current trends in LTE are for the UE not tostore a neighbor cell list, and instead, let the network store thatinformation. The signal detector 77 receives current cell identifierinformation (shown conceptually with a dashed line) to permit detectionof the information transmitted from the associated base station based onthat current cell identifier information.

Given that cell identifier collision detection, resolution, and changenotification messages are distributed to multiple base stations, thosemessages sent between the cells may travel different paths depending onthe technology. FIG. 15 shows some non-limiting examples. With LTE, themessages may travel over the X2 interface, and if that is not possible,over the S1 interface. For an Inter-Radio Access Technology (IRAT) case,the message must travel via the MIME and serving GPRS support node(SGSN) to the final controller (e.g., a BSC or an RNC). An alternativeto IRAT is that LTE informs an operational support system (OSS) about anMCI change. The OSS can then execute the necessary change via amanagement interface.

This technology provides automatic resolution of cell identifiercollisions, which reduces the burden for network operators and alsoimproves service to end users. Cell identifier planning for new cell mayalso be eliminated for new cells because even though a new cell mightintroduce a cell identifier collision, it is resolved immediately. Othercells and UE terminals may be automatically notified ahead of time ofcell identifier changes to ensure seamless and uninterrupted service.

Although the above description is based on the technology beingimplemented in base stations, some of the cell identifier collisiondetection and cell identifier change notification functions may beimplemented in other network nodes if appropriate such as a radionetwork controller or even a core network node rather than the basestation. Alternatively, the technology could be implemented using somecombination of network nodes, e.g., divided between a base station and aradio network controller. All of the illustrated signaling messagesshould be seen as conceptual and can be implemented in numerous ways.For example, all signaling could be implemented as one generic MCIconflict resolution message with different content.

None of the above description should be read as implying that anyparticular element, step, range, or function is essential such that itmust be included in the claims scope. The scope of patented subjectmatter is defined only by the claims. The extent of legal protection isdefined by the words recited in the allowed claims and theirequivalents. All structural and functional equivalents to the elementsof the above-described preferred embodiment that are known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Moreover, itis not necessary for a device or method to address each and everyproblem sought to be solved by the present invention, for it to beencompassed by the present claims. No claim is intended to invokeparagraph 6 of 35 USC §112 unless the words “means for” or “step for”are used. Furthermore, no embodiment, feature, component, or step inthis specification is intended to be dedicated to the public regardlessof whether the embodiment, feature, component, or step is recited in theclaims.

1. A method implemented in a cellular radio communications network,comprising: detecting a need for a cell identifier associated with oneof the previously-operational cells in the cellular radio communicationsnetwork that is already supporting one or more UE call connections;determining neighbor cell relations (NCR) information from cell(s)neighboring the one previously-operational cell; based on the NCRinformation, determining a cell identifier for the onepreviously-operational cell that is different from cell identifiersindicated in the NCR information; and providing the different cellidentifier to another cell different from the one previously-operationalcell.
 2. The method of claim 1, wherein the method is distributed andimplemented in more than one radio network nodes.
 3. The method of claim1, further comprising receiving information from one or more userequipment (UE) terminals regarding transmissions received from multiplecells including the same cell identifier.
 4. The method of claim 1,wherein the NCR information includes information regarding cellidentifiers for other cells.
 5. The method of claim 4, wherein the cellidentifiers for cells different from the one previously-operational cellare each associated with a corresponding signature sequence.
 6. Themethod of claim 1, further comprising: detecting that a first cellidentifier associated with a first cell is the same as a second cellidentifier associated with a second cell, and selecting one of the firstand second cells to be the one previously-operational cell and to changeits associated cell identifier.
 7. The method of claim 6, wherein a cellidentifier change message sent to a cell neighboring the selected cellincludes a time parameter from a receiving cell to indicate when tochange the corresponding cell identifier for the selected cell to thedifferent cell identifier.
 8. The method of claim 1, further comprisingone or more user equipment terminals (UEs) being served by the onepreviously-operational cell performing measurements on the cellidentifier for the one previously-operational cell when the onepreviously-operational cell discontinues transmission of a cellidentifier signature sequence for the one previously-operational cell.9. The method of claim 8, wherein one or more of the UEs reports to theone previously-operational cell a detection of the one identifier forthe one previously-operational cell during a time when the onepreviously-operational cell discontinues transmissions of the cellidentifier signature sequence for the one previously-operational cell.10. The method of claim 1, wherein the cell identifiers are reportingcell identifiers used by UE terminals to identify cells associated witha reporting parameter provided by the UE terminals in measurementreports sent by the UE terminals to the cellular radio communicationsnetwork, the method further comprising providing UE terminals with thedifferent cell identifier for the one previously-operational cell. 11.The method of claim 1, wherein the cellular radio communications networkis a long term evolution (LTE) network and the cell identifiers aremeasuring cell identifiers (MCIs) used by UE terminals to identify cellsassociated with a measurement parameter detected by the UE terminals inmeasurement reports sent by the UE terminals to the LIE network.
 12. Themethod of claim 6, wherein the selected cell is selected based on one ormore of the following decision points: a lowest cell identifier at thecellular radio communications network level, a shortest neighbor cellrelations (NCR) list, most recently changed cell identifier, a youngestcell, a lowest cell type, or a smallest maximum power.
 13. An apparatusimplemented in a cellular radio communications network that includesmultiple previously-operational cells that is already supporting one ormore UE call connections, comprising electronic circuitry configured toperform the following tasks: determine neighbor cell relations (NCR)information from one or more cells neighboring a selected one of thepreviously-operational multiple cells; based on the NCR information,determine a different cell identifier for the selected cell that isdifferent from cell identifiers indicated in the NCR information; andprovide the different cell identifier to one or more other cells. 14.The apparatus of claim 13, wherein a detecting cell includes electroniccircuitry configured to perform the detect and select tasks, and theselected cell includes electronic circuitry configured to perform thedetermine and provide tasks.
 15. The apparatus of claim 13, wherein theelectronic circuitry is configured to receive information from one ormore user equipment (UE) terminals regarding transmissions received fromfirst and second ones of the multiple cells including the same cellidentifier.
 16. The apparatus of claim 13, wherein the electroniccircuitry is configured to receive information regarding cellidentifiers for other cells.
 17. The apparatus of claim 16, wherein thecell identifiers for other cells are each associated with acorresponding signature sequence.
 18. The apparatus of claim 13implemented in a system of multiple cells, wherein a first cell informsone or more user equipment terminals (UEs) served in the first cell of adiscontinued transmission time period during which the first celldiscontinues transmission of a first cell identifier and during whichthe one or more UEs are requested to determine whether the first cellidentifier is broadcasted by another cell.
 19. The apparatus of claim13, wherein the electronic circuitry is configured to: detect that afirst cell identifier associated with a first one of the multiple cellsis the same as a second cell identifier associated with a second one ofthe multiple cells, and select one of the first and second cells to bethe selected cell and change its corresponding cell identifier.
 20. Theapparatus of claim 19, wherein a cell identifier change message sent tothe one or more other cells neighboring the selected cell includes atime parameter from a receiving cell to indicate when to change the cellidentifier for the selected cell to the different cell identifier. 21.The apparatus of claim 13, wherein the electronic circuitry isdistributed at multiple radio network nodes.
 22. The apparatus of claim13, wherein the cellular radio communications network is a long termevolution (LTE) network and the first and second cell identifiers aremeasuring cell identifiers (MCIs) used by UE terminals to identify cellsassociated with a measurement parameter detected by the UE terminals inmeasurement reports sent by the UE terminals to the LTE network.
 23. Theapparatus of claim 13, wherein the selection is based on one or more ofthe following decision points: a lowest cell identifier at the cellularradio communications network level, a shortest neighbor cell relations(NCR) list, most recently changed cell identifier, a youngest cell, alowest cell type, or a smallest maximum power.
 24. The apparatus ofclaim 13, wherein the electronic circuitry is configured to provide UEterminals the different cell identifier for the selected cell.
 25. Theapparatus of claim 13, wherein the electronic circuitry is configured todetermine an initial cell identifier for the one cell.
 26. The method ofclaim 1, wherein the detecting the need for a cell identifier associatedwith the one cell includes determining an initial cell identifier forthe one cell.